Although the invention has general applicability to any given system where the formation and deposition of calcium phosphate is a potential problem, the invention will be discussed in detail as it concerns cooling water and boiler water systems.
The term "cooling water" is applied wherever water is circulated through equipment to absorb and carry away heat. This definition includes air conditioning systems, engine jacket systems, refrigeration systems as well as the multitude of industrial heat exchange operations, such as found in oil refineries, chemical plants, steel mills, etc.
The once-through system, as the name implies, is one in which the water is passed through the heat exchange equipment and the cooling water is then discharged to waste. Usually, a once-through system is employed only where water at suitably low temperature is readily available in large volume and at low cost. The usual source of once-through cooling water is from wells, rivers and lakes where the cost involved is that of pumping only. In a once-through system, no evaporation takes place and consequently the water does not concentrate. Circulating water characteristics are the same as the makeup water.
The use of a recirculating system, in which a cooling tower, spray pond, evaporative condenser and the like serve to dissipate heat, permits great economy in makeup water requirements. With dwindling supplies of fresh cold water available for industry's cooling requirements, increased use must be made of recirculating systems in which the cooling water is used over and over again.
After passage of the circulating water through the heat exchange equipment, the water is cooled in passing over the cooling tower. This cooling effect is produced by evaporation of a portion of the circulating water in passing over the tower. By virtue of the evaporation which takes place in cooling, the dissolved solids and suspended solids in the water become concentrated.
The circulating water becomes more concentrated than the makeup water due to this evaporation loss. Cycles of concentration is the term employed to indicate the degree of concentration of the circulating water as compared with the makeup. For example, 2.0 cycles of concentration indicates the circulating water is twice the concentration of the makeup water.
Deposits in lines, heat exchange equipment, etc., may originate from several causes. For example, the precipitation of calcium carbonate and calcium phosphate will form scale, but products of corrosion also result in a deposit of iron oxide. In speaking of deposits which form in cooling water systems, it is important to bear in mind the mechanism causing the deposit, otherwise confusion may result. In general, the term "scale" applies to deposits which result from crystallization or precipitation of salts from solution. Wasting away of a metal is the result of corrosion. While a deposit results in both cases, the mechanism of formation is different and different corrective methods are required to prevent the deposit.
Some of the factors which affect scale formation are temperature, rate of heat transfer, the calcium, sulfate, magnesium, silica, phosphate, alkalinity, dissolved solids and pH of the water.
In the past in order to minimize the formation of the scale forming salts, the cooling water systems were operated at pH's where the solubility of the "hardness" or "scale forming" ions were the greatest. Because the pH's of the systems were acidic, corrosion inhibitors together with dispersants were the normal treatment. Corrosion inhibition in most instances required chromate treatment. With the advent of tight controls as regards toxic pollutant discharge, operating parameters of cooling water systems had to be changed in an attempt to utilize nonchromate treatment. The development of high pH and/or non-chromate corrosion programs over the past few years has concurrently enhanced the potential for heat exchange fouling due to chemical precipitation. Since most of the treatments currently used include phosphate and/or phosphonic acid compounds, such as the alkali metal polyphosphates, organo-phosphates, e.g., phosphate esters, etc., amino-trimethylene phosphonic acid, hydroxy ethylidene diphosphonic acid, and the water soluble salts thereof and since there may be phosphate in the makeup water supply for example tertiary sewage treatment effluent for makeup water, calcium phosphate scaling has become one of the major problems encountered. The reversion of the polyphosphates and the organic phosphates plus the use of alkaline operating conditions leads to the formation and deposition of the highly insoluble calcium phosphate.
Although steam generating systems are somewhat different from cooling water systems, they share a common problem as relates for example to calcium phosphate and iron oxide formation and deposition.
As detailed in the Betz Handbook of Industrial Water Conditioning, 6th Edition, 1968, Betz Laboratories, Inc., Trevose, Pa., pages 151-171, the formation of scale and sludge deposits on boiler heating surfaces is the most serious water problem encountered in steam generation. Although current industrial steam producing systems make use of sophisticated external treatments of the boiler feed water, e.g., coagulation, filtration, softening of water prior to its feed into the boiler system, those operations are only moderately effective. In all cases, external treatment does not in itself provide adequate treatment since muds, sludge, silts and hardness-imparting ions escape the treatment, and eventually are introduced into the steam generating system.
The problems which result from their introduction into the steam generating system are apparent. Since the deposit forming materials are present, they have a tendency to accumulate upon concentration of the water and to settle at points in the system where there is low flow and therefore to restrict water circulation. The baking of mud and/or sludge on tubes and sheets will result in overheating and failure, thereby requiring down time for repair or replacement of the structural parts. In addition, mud, sludge and silts may become incorporated in scale deposits adding to their volume and heat insulating effect.
Accordingly, internal treatments have been necessary to maintain the mud and silts in a suspended state. These internal treatments have been generally referred to in the industry as sludge conditioning agents.
In addition to the problems caused by mud, sludge or silts, the industry has also had to contend with boiler scale. Although external treatment is utilized specifically in an attempt to remove calcium and magnesium from the feed water, scale formation due to residual hardness, i.e., calcium and magnesium salts, is always experienced. Accordingly, internal treatment, i.e., treatment of the water fed to the system, is necessary to prevent, reduce and/or retard formation of the scale-imparting compounds and their deposition. The carbonates of magnesium and calcium are not the only problem compounds as regards scale, but also waters having high contents of phosphate, sulfate and silicate ions either occurring naturally or added for other purposes are problematic since calcium and magnesium, and any iron or copper present, react with each and deposit as boiler scale. As is obvious, the deposition of scale on the structural parts of a steam generating system causes poorer circulation and lower heat transfer capacity, resulting accordingly in an overall loss in efficiency.
Although the foregoing is directed for the most part to cooling water systems and boiler water systems, or more specifically steam generating systems, the same problems occur in scrubber systems and the like. Any aqueous system having calcium and magnesium cations and the exemplified anions, in particular phosphate, will experience the formation and deposition of the scaling salts.
Because of the foregoing, the water treatment industry is constantly evaluating new processes, new products, new techniques in an effort to permit the various process water systems to operate more effectively for longer periods and at lower costs.
Many and different type materials have been used for the treatment of water systems. Of the vast number may be mentioned alginates, lignins, lignosulfonates, tannins, carboxymethyl cellulose materials, and synthetic polymers such as polyacrylates and polymethacrylates.